Combined closed loop/open loop power control in a time division duplex communication system

ABSTRACT

A spread spectrum time division duplex user equipment uses frames having time slots for communication. The user equipment receives power commands and a first communication having a transmission power level in a first time slot. A power level of the first communication is measured as received. A pathloss estimate is determined based on in part the measured received first communication power level and the first communication transmission power level. A transmission power level for the second communication in a second time slot form the user equipment is set based on in part the pathloss estimate weighted by a quality factor adjusted by the power command. The quality factor decreases as a number of time slots between the first and second time slots increases.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/531,359 filed Mar. 21, 2000, which is incorporated byreference as if fully set forth.

BACKGROUND

[0002] This invention generally relates to spread spectrum time divisionduplex (TDD) communication systems. More particularly, the presentinvention relates to a system and method for controlling transmissionpower within TDD communication systems.

[0003]FIG. 1 depicts a wireless spread spectrum time division duplex(TDD) communication system. The system has a plurality of base stations301-307. Each base station 301 communicates with user equipments (UEs)321-323 in its operating area. Communications transmitted from a basestation 301 to a UE 321 are referred to as downlink communications andcommunications transmitted from a UE 321 to a base station 301 arereferred to as uplink communications.

[0004] In addition to communicating over different frequency spectrums,spread spectrum TDD systems carry multiple communications over the samespectrum. The multiple signals are distinguished by their respectivechip code sequences (codes). Also, to more efficiently use the spreadspectrum, TDD systems as illustrated in FIG. 2 use repeating frames 34divided into a number of time slots 361-36 n, such as fifteen timeslots. In such systems, a communication is sent in selected time slots361-36 n using selected codes. Accordingly, one frame 34 is capable ofcarrying multiple communications distinguished by both time slot 361-36n and code. The combination of a single code in a single time slot isreferred to as a resource unit. Based on the bandwidth required tosupport a communication, one or multiple resource units are assigned tothat communication.

[0005] Most TDD systems adaptively control transmission power levels. Ina TDD system, many communications may share the same time slot andspectrum. When a UE 321 or base station 301 is receiving a specificcommunication, all the other communications using the same time slot andspectrum cause interference to the specific communication. Increasingthe transmission power level of one communication degrades the signalquality of all other communications within that time slot and spectrum.However, reducing the transmission power level too far results inundesirable signal to noise ratios (SNRs) and bit error rates (BERs) atthe receivers. To maintain both the signal quality of communications andlow transmission power levels, transmission power control is used.

[0006] One approach to control transmission power levels is open looppower control. In open loop power control, typically a base station 301transmits to a UE 321 a reference downlink communication and thetransmission power level of that communication. The UE 321 receives thereference communication and measures its received power level. Bysubtracting the received power level from the transmission power level,a pathloss for the reference communication is determined. To determine atransmission power level for the uplink, the downlink pathloss is addedto a desired received power level at the base station 301. The UE'stransmission power level is set to the determined uplink transmissionpower level.

[0007] Another approach to control transmission power level is closedloop power control. In closed loop power control, typically the basestation 301 determines the signal to interference ratio (SIR) of acommunication received from the UE 321. The determined SIR is comparedto a target SIR (SIRTARGET). Based on the comparison, the base station301 transmits a power command, bTPC. After receiving the power command,the UE 321 increases or decreases its transmission power level based onthe received power command.

[0008] Both closed loop and open loop power control have disadvantages.Under certain conditions, the performance of closed loop systemsdegrades. For instance, if communications sent between a UE and a basestation are in a highly dynamic environment, such as due to the UEmoving, such systems may not be able to adapt fast enough to compensatefor the changes. The update rate of closed loop power control in TDD is100 cycles per second which is not sufficient for fast fading channels.Open loop power control is sensitive to uncertainties in the uplink anddownlink gain chains and interference levels.

[0009] One approach to combining closed loop and open loop power controlwas proposed by the Association of Radio Industries and Business (ARIB)and uses Equations 1, 2, and 3.

T _(UE) =P _(BS)(n)+L  Equation 1

P _(BS)(n)=P _(BS)(n−1)+b _(TPC)Δ_(TPC)  Equation 2 $\begin{matrix}{b_{TPC} = \left\{ \begin{matrix}{1:\quad {{if}\quad {SIR}_{BS}{\langle{SIR}_{TARGET}}}} \\{{{{{- 1}:\quad {{if}\quad {SIR}_{BS}}}\rangle}{SIR}_{TARGET}}\quad}\end{matrix} \right.} & {{Equation}\quad 3}\end{matrix}$

[0010] T_(UE) is the determined transmission power level of the UE 32 ₁.L is the estimated downlink pathloss. PBs(n) is the desired receivedpower level of the base station 30 ₁ as adjusted by Equation 2. For eachreceived power command, b_(TPC), the desired received power level isincreased or decreased by Δ_(TPC). Δ_(TPC) is typically one decibel(dB). The power command, b_(TPC), is one, when the SIR of the UE'suplink communication as measured at the base station 30, SIRBS, is lessthan a target SIR, SIR_(TARGET). Conversely, the power command is minusone, when SIR_(BS) is larger than SIR_(TARGET).

[0011] Under certain conditions, the performance of these systemsdegrades. For instance, if communications sent between a UE 32 and abase station 30 are in a highly dynamic environment, such as due to theUE 32 moving, the path loss estimate for open loop severely degrades theoverall system's performance. Accordingly, there is a need for alternateapproaches to maintain signal quality and low transmission power levelsfor all environments and scenarios.

[0012] SUMMARY

[0013] Combined closed loop/open loop power control controlstransmission power levels in a spread spectrum time division duplexcommunication station. A first communication station receivescommunications from a second communication station. The first stationtransmits power commands based on in part a reception quality of thereceived communications. The first station transmits a secondcommunication having a transmission power level in a first time slot.The second station receives the second communication and the powercommands. A power level of the second communication as received ismeasured. A path loss estimate is determined based on in part themeasured received second communication power level and the firstcommunication transmission power level. The second station transmits asecond communication to the first station in a second time slot. Thesecond communication transmission power level is set based on in partthe path loss estimate weighted by a factor and the power commands. Thefactor is a function of a time separation of the first and second timeslots.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 illustrates a prior art TDD system.

[0015]FIG. 2 illustrates time slots in repeating frames of a TDD system.

[0016]FIG. 3 is a flow chart of combined closed loop/open loop powercontrol.

[0017]FIG. 4 is a diagram of components of two communication stationsusing combined closed loop/open loop power control.

[0018] FIGS. 5-10 depict graphs of the performance of a closed loop,ARIB's proposal and two (2) schemes of combined closed loop/open looppower control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The preferred embodiments will be described with reference to thedrawing figures where like numerals represent like elements throughout.Combined closed loop/open loop power control will be explained using theflow chart of FIG. 3 and the components of two simplified communicationstations 50, 52 as shown in FIG. 4. For the following discussion, thecommunication station having its transmitter's power controlled isreferred to as the transmitting station 52 and the communication stationreceiving power controlled communications is referred to as thereceiving station 50. Since combined closed loop/open loop power controlmay be used for uplink, downlink or both types of communications, thetransmitter having its power controlled may be located at a base station30 ₁, UE 32 ₁ or both. Accordingly, if both uplink and downlink powercontrol are used, the receiving and transmitting station's componentsare located at both the base station 30 ₁ and UE 32 ₁.

[0020] The receiving station 50 receives various radio frequency signalsincluding communications from the transmitting station 52 using anantenna 56, or alternately, an antenna array. The received signals arepassed through an isolator 60 to a demodulator 68 to produce a basebandsignal. The baseband signal is processed, such as by a channelestimation device 96 and a data estimation device 98, in the time slotsand with the appropriate codes assigned to the transmitting station'scommunication. The channel estimation device 96 commonly uses thetraining sequence component in the baseband signal to provide channelinformation, such as channel impulse responses. The channel informationis used by the data estimation device 98, the interference measurementdevice 90, the signal power measurement device 92 and the transmit powercalculation device 94. The data estimation device 98 recovers data fromthe channel by estimating soft symbols using the channel information.Using the soft symbols and channel information, the transmit powercalculation device 94 controls the receiving station's transmissionpower level by controlling the gain of an amplifier 76.

[0021] The signal power measurement device 92 uses either the softsymbols or the channel information, or both, to determine the receivedsignal power of the communication in decibels (dB). The interferencemeasurement device 90 determines the interference level in dB, I_(RS),within the channel, based on either the channel information, or the softsymbols generated by the data estimation device 98, or both.

[0022] The closed loop power command generator 88 uses the measuredcommunication's received power level and the interference level, IRS, todetermine the Signal to Interference Ratio (SIR) of the receivedcommunication. Based on a comparison of the determined SIR with a targetSIR (SIR_(TARGET)), a closed loop power command is generated, b_(TPC),such as a power command bit, b_(TPC), step 38. Alternately, the powercommand may be based on any quality measurement of the received signal.

[0023] For use in estimating the path loss between the receiving andtransmitting stations 50, 52 and sending data, the receiving station 50sends a communication to the transmitting station 58, step 40. Thecommunication may be sent on any one of various channels. Typically, ina TDD system, the channels used for estimating path loss are referred toas reference channels, although other channels may be used. If thereceiving station 50 is a base station 30 ₁, the communication ispreferably sent over a downlink common channel or a common controlphysical channel (CCPCH). Data to be communicated to the transmittingstation 52 over the reference channel is referred to as referencechannel data. The reference data may include, as shown, the interferencelevel, IRS, multiplexed with other reference data, such as thetransmission power level of the reference channel, T_(RS). Theinterference level, IRS, and reference channel power level, T_(RS), maybe sent in other channels, such as a signaling channel. The closed looppower control command, b_(TPC), is typically sent in a dedicatedchannel. The dedicated channel is dedicated to the communication betweenthe receiving station 50 and transmitting station 52, step 40.

[0024] The reference channel data is generated by a reference channeldata generator 86. The reference data is assigned one or multipleresource units based on the communication's bandwidth requirements. Aspreading and training sequence insertion device 82 spreads thereference channel data and makes the spread reference datatime-multiplexed with a training sequence in the appropriate time slotsand codes of the assigned resource units. The resulting sequence isreferred to as a communication burst. The communication burst issubsequently amplified by an amplifier 78. The amplified communicationburst may be summed by a sum device 72 with any other communicationburst created through devices, such as a data generator 84, spreadingand training sequence insertion device 80 and amplifier 76.

[0025] The summed communication bursts are modulated by a modulator 64.The modulated signal is passed through an isolator 60 and radiated by anantenna 56 as shown or, alternately, through an antenna array. Theradiated signal is passed through a wireless radio channel 54 to anantenna 58 of the transmitting station 52. The type of modulation usedfor the transmitted communication can be any of the those known to thoseskilled in the art, such as direct phase shift keying (DPSK) orquadrature phase shift keying (QPSK).

[0026] The antenna 58 or, alternately, antenna array of the transmittingstation 52 receives various radio frequency signals. The receivedsignals are passed through an isolator 62 to a demodulator 66 to producea baseband signal. The baseband signal is processed, such as by achannel estimation device 100 and a data estimation device 102, in thetime slots and with the appropriate codes assigned to the communicationburst of the receiving station 50. The channel estimation device 100commonly uses the training sequence component in the baseband signal toprovide channel information, such as channel impulse responses. Thechannel information is used by the data estimation device 102, a powermeasurement device 110 and a quality measurement device 114.

[0027] The power level of the processed communication corresponding tothe reference channel, R_(TS), is measured by the power measurementdevice 110 and sent to a pathloss estimation device 112, step 42. Boththe channel estimation device 100 and the data estimation device 102 arecapable of separating the reference channel from all other channels. Ifan automatic gain control device or amplifier is used for processing thereceived signals, the measured power level is adjusted to correct forthe gain of these devices at either the power measurement device 110 orthe pathloss estimation device 112. The power measurement device 110 isa component of the combined closed loop/open loop controller 108. Asillustrated in FIG. 4, the combined closed loop/open loop powercontroller 108 comprises the power measurement device 110, pathlossestimation device 112, quality measurement device 114, and transmitpower calculation device 116.

[0028] To determine the path loss, L, the transmitting station 52 alsorequires the communication's transmitted power level, T_(RS). Thetransmitted power level, T_(RS), may be sent along with thecommunication's data or in a signaling channel. If the power level,T_(RS), is sent along with the communication's data, the data estimationdevice 102 interprets the power level and sends the interpreted powerlevel to the pathloss estimation device 112. If the receiving station 50is a base station 30 ₁, preferably the transmitted power level, T_(RS),is sent via the broadcast channel (BCH) from the base station 30 ₁. Bysubtracting the received communication's power level, RTS in dB, fromthe sent communication's transmitted power level, TRS in dB, thepathloss estimation device 112 estimates the path loss, L, between thetwo stations 50, 52, step 44. In certain situations, instead oftransmitting the transmitted power level, T_(RS), the receiving station50 may transmit a reference for the transmitted power level. In thatcase, the pathloss estimation device 112 provides reference levels forthe path loss, L.

[0029] If a time delay exists between the estimated path loss and thetransmitted communication, the path loss experienced by the transmittedcommunication may differ from the calculated loss. In TDD systems wherecommunications are sent in differing time slots 36 ₁-36 _(n), the timeslot delay between received and transmitted communications may degradethe performance of an open loop power control system. Combined closedloop/open loop power control utilizes both closed loop and open looppower control aspects. If the quality of the path loss measurement ishigh, the system primarily acts as an open loop system. If the qualityof the path loss measurement is low, the system primarily acts as aclosed loop system. To combine the two power control aspects, the systemweights the open loop aspect based on the quality of the path lossmeasurement.

[0030] A quality measurement device 114 in a weighted open loop powercontroller 108 determines the quality of the estimated path loss, step46. The quality may be determined using the channel informationgenerated by the channel estimation device 100, the soft symbolsgenerated by the data estimation device 102 or other quality measurementtechniques. The estimated path loss quality is used to weight the pathloss estimate by the transmit power calculation device 116. If the powercommand, b_(TPC), was sent in the communication's data, the dataestimation device 102 interprets the closed loop power command, b_(TPC).Using the closed loop power command, b_(TPC), and the weighted pathloss, the transmit power calculation device 116 sets the transmit powerlevel of the receiving station 50, step 48.

[0031] The following is one of the preferred combined closed loop/openloop power control algorithms. The transmitting station's power level indecibels, PTS, is determined using Equations 4 and 6.

P _(TS) =P ₀ +G(n)+αL  Equation 4

[0032] P₀ is the power level that the receiving station 50 desires toreceive the transmitting station's communication in dB. P₀ is determinedby the desired SIR at the receiving station 50, SIRTARGET, and theinterference level, IRS, at the receiving station 50 using Equation 5.

P ₀ =SIR _(TARGET) +I _(RS)  Equation 5

[0033] IRS is either signaled or broadcasted from the receiving station50 to the transmitting station 52. For downlink power control,SIR_(TARGET) is known at the transmitting station 52. For uplink powercontrol, SIR_(TARGET) is signaled from the receiving station 50 to thetransmitting station 52. G(n) is the closed loop power control factor.Equation 6 is one equation for determining G(n).

G(n)=G(n−1)+b _(TPC)Δ_(TPC)  Equation 6

[0034] G(n−1) is the previous closed loop power control factor. Thepower command, b_(TPC), for use in Equation 6 is either +1 or −1. Onetechnique for determining the power command, b_(TPC), is Equation 3. Thepower command, b_(TPC), is typically updated at a rate of 100 ms in aTDD system, although other update rates may be used. Δ_(TPC) is thechange in power level. The change in power level is typically 1 dBalthough other values may be used. As a result, the closed loop factorincreases by 1 dB if bTPC is +1 and decreases by 1 dB if bTPC is −1.

[0035] The weighting value, a, is determined by the quality measurementdevice 114. a is a measure of the quality of the estimated path loss andis, preferably, based on the number of time slots, D, between the timeslot of the last path loss estimate and the first time slot of thecommunication transmitted by the transmitting station 52. The value of ais from zero to one. Generally, if the time difference, D, between thetime slots is small, the recent path loss estimate will be fairlyaccurate and a is set at a value close to one. By contrast, if the timedifference is large, the path loss estimate may not be accurate and theclosed loop aspect is most likely more accurate. Accordingly, a is setat a value closer to zero.

[0036] Equations 7 and 8 are two equations for determining a, althoughothers may be used.

α=1−(D−1)/(D _(max)−1)  Equation 7

α=max{1−(D−1)/(D _(max-allowed)−1), 0}  Equation 8

[0037] D_(max) is the maximum possible delay. A typical value for aframe having fifteen time slots is seven. If the delay is D_(max), a iszero. D_(max-allowed) is the maximum allowed time slot delay for usingopen loop power control. If the delay exceeds D_(max-allowed), open looppower control is effectively turned off by setting α=0. Using thecalculated transmit power level, P_(TS), determined by a transmit powercalculation device 116, the combined closed loop/open loop powercontroller 108 sets the transmit power of the transmitted communication.

[0038] Data to be transmitted in a communication from the transmittingstation 52 is produced by a data generator 106. The communication datais spread and time-multiplexed with a training sequence by the spreadingand training sequence insertion device 104 in the appropriate time slotsand codes of the assigned resource units producing a communicationburst. The spread signal is amplified by the amplifier 74 and modulatedby the modulator 70 to radio frequency.

[0039] The combined closed loop/open loop power controller 108 controlsthe gain of the amplifier 74 to achieve the determined transmit powerlevel, PTS, for the communication. The power controlled communication ispassed through the isolator 62 and radiated by the antenna 58.

[0040] Equations 9 and 10 are another preferred combined closedloop/open loop power control algorithm.

P _(TS) =P ₀ +K(n)  Equation 9

K(n)=K(n−1)+b _(TPC)Δ_(TPC) +αL  Equation 10

[0041] K(n) is the combined closed loop/open loop factor. As shown, thisfactor includes both the closed loop and open loop power controlaspects. Equations 4 and 5 segregate the two aspects.

[0042] Although the two above algorithms only weighted the open loopfactor, the weighting may be applied to the closed loop factor or boththe open and closed loop factors. Under certain conditions, the networkoperator may desire to use solely open loop or solely closed loop powercontrol. For example, the operator may use solely closed loop powercontrol by setting a to zero.

[0043] FIGS. 5-10 depict graphs 118-128 illustrating the performance ofa combined closed-loop/open-loop power control system. These graphs118-128 depict the results of simulations comparing the performance ofthe ARIB proposed system, a closed loop, a combined open loop/closedloop system using Equations 4 and 6 (scheme I) and a combined systemusing Equations 9 and 10 (scheme II). The simulations were performed atthe symbol rate. A spreading factor of sixteen was used for both theuplink and downlink channels. The uplink and downlink channels areInternational Telecommunication Union (ITU) Channel model [ITU-R M.1225,vehicular, type B]. Additive noises were simulated as being independentof white Gaussian noises with unity variance. The path loss is estimatedat the transmitting station 52 which is a UE 32 ₁ and in particular amobile station. The BCH channel was used for the path loss estimate. Thepath loss was estimated two times per frame at a rate of 200 cycles persecond. The receiving station 50, which was a base station 30 ₁, sentthe BCH transmission power level over the BCH. RAKE combining was usedfor both the UE 32 ₁ and base station 30 ₁. Antenna diversity combiningwas used at the base station 30 ₁.

[0044] Graphs 118, 122, 126 depict the standard deviation of thereceived signal to noise ratio (SNR) at the base station 30 ₁ of theUE's power controlled communication as a function of the time slotdelay, D. Graphs 120, 124, 128 depict the normalized bias of thereceived SNR as a function of the delay, D. The normalization wasperformed with respect to the desired SNR. Each point in the graphs118-128 represents the average of 3000 Monte-Carlo runs.

[0045] Graphs 118, 120 depict the results for an a set at one. For lowtime slot delays (D<4), scheme I and II outperform closed loop powercontrol. For larger delays (D≧4), closed loop outperforms both scheme Iand II which demonstrates the importance of weighting the open loop andclosed loop aspects.

[0046] Graphs 122, 124 depict the results for an α set at 0.5. As shown,for all delays excluding the maximum, schemes I and II outperform closedloop power control. The ARIB proposal only outperforms the others at thelowest delay (D=1).

[0047] Graphs 126, 128 depict the results for an a set using Equation 7with D_(max) equal to seven. As shown, schemes I and II outperform bothclosed loop and the ARIB proposal at all delays, D.

What is claimed is:
 1. A spread spectrum time division duplex userequipment, the user equipment using frames with time slots forcommunication, receiving power commands and receiving a firstcommunication having a transmission power level in a first time slot,measuring a power level of the first communication as received anddetermining a pathloss estimate based on in part the measured receivedfirst communication power level and the first communication transmissionpower level, the user equipment comprising: means for setting atransmission power level for a second communication in a second timeslot from the user equipment based on in part the pathloss estimateweighted by a quality factor adjusted by the power commands, wherein thequality factor decreases as a number of time slots between the first andsecond time slots increases.
 2. The user equipment of claim 1 furthercomprising means for determining the quality factor, a, of the pathlossestimate based on in part a number of time slots, D, between the firstand second time slot.
 3. The user equipment of claim 2 wherein a maximumtime slot delay is D_(max) and the determined quality factor, α, isdetermined by α=1−(D−1)/D _(max).
 4. The user equipment of claim 2wherein the setting means sets the transmission power level based on inpart a desired received power level, a closed loop factor and an openloop factor, the closed loop factor is based on in part the receivedpower commands and the open loop factor is based on in part the pathlossestimate weighted by the quality factor.
 5. The user equipment of claim2 wherein the setting means sets the transmission power level based onin part a desired received power level at the first station and acombined closed loop/open loop factor, the combined closed loop/openloop factor is based on in part the received power commands and the pathloss estimate weighted by the quality factor.
 6. The user equipment ofclaim 4 wherein the closed loop factor is updated for each receivedpower command.
 7. The user equipment of claim 5 wherein the combinedfactor is updated for each received power command.
 8. The user equipmentof claim 4 wherein the desired received power level is based on in parta target signal to interference ratio and a measured interference level.9. The user equipment of claim 5 wherein the desired received powerlevel is based on in part a target signal to interference ratio and ameasured interference level.